The present invention relates to the use of hydroxyl-containing tertiary amines as catalysts for producing polyurethanes.
Polyurethane foams are widely known and used in the automotive, housing, and other industries. Such foams are produced by reaction of a polyisocyanate with a polyol in the presence of various additives. One such additive is a chlorofluorocarbon (CFC) blowing agent which vaporizes as a result of the reaction exotherm causing the polymerizing mass to form a foam. The discovery that CFCs deplete ozone in the stratosphere has resulted in mandates diminishing CFC use. Production of water blown foams, in which blowing is performed with CO2 generated by the reaction of water with the polyisocyanate, has therefore become increasingly important. Tertiary amine catalysts are typically used to accelerate blowing, for example, the reaction of water with polyisocyanate to generate CO2, and gelling, for example, the reaction of a polyol with isocyanate.
The ability of the tertiary amine catalyst to selectively promote either blowing or gelling is an important consideration in selecting a catalyst for the production of a particular polyurethane foam. If a catalyst promotes the blowing reaction to an excessive degree, much of the CO2 will be evolved before sufficient reaction of isocyanate with polyol has occurred, and the CO2 will bubble out of the formulation, resulting in a collapse of the polymerization mass and yielding foam of poor quality. In contrast, if a catalyst too strongly promotes the gelling reaction, a substantial portion of the CO2 will be evolved after a significant degree of polymerization has occurred. Again, poor quality foams which are characterized by high density, broken or poorly defined cells, or other undesirable features will be produced.
Tertiary amine catalysts generally are malodorous and offensive and many have high volatility due to their low molecular weight. Release of tertiary amines during foam processing may present significant safety and toxicity problems, and release of residual amines from customer products is generally undesirable. On the other hand, low vapor pressure/high molecular weight amine catalysts are expected to require very high catalyst usage due to their low nitrogen/carbon ratio, making the manufacturing cost too high.
Tertiary amine catalysts containing hydroxyl functionality can chemically bind into the urethane matrix during the polymerization reaction, thus limiting their release from the finish product. For example, amine catalysts which contain primary and/or secondary hydroxyl functionality typically have limited volatility and low odor when compared to related structures without hydroxyl functionality.
The most effective hydroxyl-containing amine catalysts are the ones containing secondary alcohols in their structures because these catalysts exhibit a desirable balance between their promotion of the active hydrogen-isocyanate reaction (gelling and blowing) and their own reactivity with isocyanates. In addition, because the primary alcohols described in the prior art are highly reactive with isocyanates, they tend to lose their catalytic activity too early in the polymerization process. As a result, polyurethane foams produced using the primary alcohols described in the art have large voids and exhibit decreased demoldability. Furthermore, large amounts of the catalyst compositions containing primary hydroxyl functionality are required to effectively catalyze the blowing reaction. See for example, U.S. Pat. Nos. 4,026,840, 5,508,314; 5,559,161; and 5,633,293. However, tertiary amine catalysts containing primary hydroxyl functionality are advantageous because they are more efficiently immobilized into the polyurethane matrix before and after polymerization. In addition, tertiary amines containing primary hydroxyl groups are characterized by lower odor and vapor pressure than their secondary hydroxyl counterparts. It is therefore desirable to produce a tertiary amine catalyst having primary hydroxyl functionality which more effectively promotes the blowing and gelling reactions while retaining their catalytic activity during the polymerization process.